Four stroke, diesel cycle internal combustion engines are well known. One of ordinary skill in the art will readily recognize that such engines operate through four distinct strokes of a piston reciprocating within a cylinder. In an intake stroke, the piston descends within the cylinder while an intake valve is open. Air is thereby able to enter the cylinder through the open intake valve. In a subsequent compression stroke, the piston reverses direction while the intake valve and an exhaust valve are closed, thereby compressing the air. This is followed by a combustion or power stroke wherein the fuel is directly injected into the compressed air and thereby ignited, with the resulting force pushing the piston again in the descending direction while both valves are closed. Finally, the piston reverses direction with the exhaust valve open, thereby pushing the combustion gases out of the cylinder.
One known disadvantage of such engine operation is that significant combustion gas energy is lost during the exhaust blowdown stage. The Miller cycle modifies a traditional Otto or Diesel cycle to, among other things, lower the effective compression ratio, which then increases the ratio of expansion to compression work by the piston and thereby improves mechanical efficiency. For the purpose of discussion, compression ratio is defined herein as a ratio of the engine cylinder capacity when a piston therein is at a bottom dead center position, to the engine cylinder capacity when the piston is at a top dead center position.
The effective compression ratio of an engine can be lowered in at least two ways. One reduces the effective length of the engine compression stroke by opening the engine intake valve for the initial stages of the compression stroke. Since true compression cannot start until the intake valve closes, the compression ratio is necessarily reduced. Another way to reduce the effective compression ratio is to close the intake valve early when still in the later stages of the intake stroke.
While the reduction in effective engine compression ratio will allow for improved mechanical efficiency, it will also tend to reduce the total power output capacity of the engine. For example, if the engine intake valve were to be opened during the initial stages of the compression stroke, a certain volume of air would escape the engine cylinder and thereby not be available for combustion. The Miller cycle therefore may be enhanced by the use of a turbocharger. The turbocharger forces highly compressed air into the cylinder during the compression stroke to make up for such losses through the intake valve. In order to retain the power output capacity and air/fuel ratio of a previously turbocharged engine, the intake air must be compressed to even higher pressure levels when operating the Miller cycle.
A result stemming from the introduction of such turbocharged air, however, is an increase in intake air temperature. Increased intake air temperature leads to reduced intake air density and increased pollutant production such as nitrous oxide (NOx), and engine knock. The highly compressed air from the turbocharger is therefore often cooled prior to introduction to the cylinder, as by an intercooler or the like, in order to maximize air density.
Another difficulty encountered with Miller cycle engine operation, is that the intake valve, or exhaust valve, must be opened and held open against significant forces, such as, for example, forces resulting from inertial and valve spring loads.
The present disclosure is directed to overcoming one or more of the problems or disadvantages associated with the prior art.